Purchasing a grab can be more significant in calculating a price for a shipping contract than it might seem at first sight.

Many years ago, a shipping company secured a contract on the basis of a calculation that four vessels would be required to handle the required bulk volume (cement-clinker) within the given time.

After a while it was apparent that the grabs on the four vessels were working so efficiently and fast that charge and discharge times were in fact much shorter than had been expected. On re-calculation, the shipping company established that it was able to fulfil the time schedule and handle the volume with only three vessels because of the reduced time spent in harbours. In effect, the company needed only three vessels for the job but was being paid on the basis of four vessels. This story is, of course, a special case but it shows the impact that grabs can have.

Grabs are the link between the vessel and shore. Grabs (and the efficiency of The Link Between Ship and Shore Orts GmbH 1 the crane-drivers) dictate how long a vessel will stay in harbour. But how do you decide what kind of grab best suits your requirements? What kind of bulk cargo has to be handled? What kind of equipment is already in place? The deadweight and the volume of grabs have to be appropriate to the cranes with which they are used.

For this the SWL (Safe Working Load) of the crane is important as well as the kind of grab the crane is able to operate. The calculation formula to find a suitable grab is: (bulk density x grab volume) + grab deadweight < SWL of the crane.

Grab Opening and Closure Systems
Different grab opening and closure systems are available:

• Fully mechanical (mechanical rope grab: single-rope, two-rope or four-rope)
• Electro-hydraulic grab (external power supply necessary)
• Radio controlled diesel-hydraulic grab.

The clamshell-design is suitable for handling scrap, pig iron, clumpy

limestone or stones but all of the types of grab listed are also available as 'orange peel' or 'polyp' grabs. The advantages of a single-rope grab are that it can be operated on any kind of crane and, as they are quite robust, they are suitable for areas or circumstances where only low maintenance is possible.

Disadvantages are that these grabs are the 'slowest' solution and can have problems when they are working high because of the length of the closing rope.

This means that at low tide in tidal harbours grabs may not be able to reach the top of the hoppers. It also means that the length of the cycle will be greater because the jib has to be lifted much more to reach the hopper on shore.

Some manufacturers offer mechanical single-rope grabs with the option of remotely controlled grab opening but in fact this will not increase the discharge rate while increasing the possibility of downtime due to electrical and hydraulics malfunction.

The simplicity and low maintenance virtues of a single-rope grab are therefore partly lost. Two-rope and four-rope grabs need a crane with two or four winches.

In the former, one rope is the holding rope and the other opens and closes the grab. A four-rope grab works on the same principle, but with two holding ropes and two closing ropes.

The closing force of all mechanical rope grabs depends on their deadweight and how often the rope is folded inside the closing mechanism. The more often the rope is folded (as with block and tackle systems) the slower the grab closes, but the greater the power. Such grabs are effective and robust, but they can not be operated on a crane with a hook on a single line. Mechanical versus powered operation Electro-hydraulic grabs are equipped with an hydraulic-cylinder system.

They are faster than mechanical rope grabs and only need a hook, but they do require additional equipment. An electrohydraulic grab not only requires a power supply from the crane to run the electric motor that powers the hydraulic pump but also a cable drum, rope drum, cable guide rollers, turn-around pulley and electrical controls for the grab functions. A radio controlled diesel-hydraulic grab has a diesel engine instead of an electric motor.

It is not dependent on an external power supply. It is just as fast as an electro-hydraulic grab and, like a singlerope grab, it can operate on every kind of crane without any further equipment. Suitable for the material being handled What kind of bulk cargo will be handled with the grab? For heavier bulk, like iron ore, pig iron, bauxite, clinker, a heavier and more powerful grab is necessary.

The mechanical/physical behaviour of different bulk materials varies; so too will the ability of the grab's teeth to cut into the load. An inappropriate choice of grab clamshell design may mean that the material being handled will prevent the grab closing and cause it to shed part of its load.

Cost considerations
A fast, efficient and reliable grab must not only offer handling efficiencies but also be good value in terms of maintenance, repair and replacement parts. Grabs may appear to look all the same from outside, but the internal, invisible, features determine the quality, reliability and effectiveness of a grab.

What are the kinematics of the clamshell design; what stresses will be imposed on grab construction and bearings? Balanced and optimised kinematics mean that the grab uses its power at the optimum point and this in turn means bearings and the grab itself will last longer.

The more information a customer is able to give the grab manufacturer about the crane, the intended method of operation (stationary or switching from crane to crane) and the bulk material that the grab will be mainly handling, the more satisfactory the choice of grab will be.

ORTS GmbH Maschinenfabrik
Schwartauer Strasse 99
D-23611 Sereetz
Germany
Tel: +49 (0) 451 39 88 50
Fax: +49 (0) 451 39 23 74

E-mail: sigvard.orts-jun@orts-gmbh.de
Web: www.orts-greifer.de